1. Field of the Invention
The present invention relates to a method and to an apparatus for making an aerogel film which is porous, exhibits a low dielectric constant and low signal loss, and may be used as a dielectric layer in a high-frequency circuit and an insulating interlayer in a semiconductor device such as an LSI. The method and the apparatus can minimize variations in film characteristics, such as thickness, porosity, and dielectric constant.
In particular, the present invention relates to a method and to an apparatus for making an aerogel film having stable qualities which can minimize variations in film characteristics, such as thickness, porosity, and dielectric constant, among a plurality of substrates with aerogel films from one production lot when these substrates with gel films are produced by a simultaneous supercritical or sub-critical drying process so as to control gelation reactions in the wet-gel films.
2. Description of the Related Art
Microwave circuits for portable phones, satellite broadcasts, and telecommunications employ integrated circuits using microstrips instead of conventional waveguides or coaxial cables.
With reference to FIG. 1A, a single-layered microstrip includes a substrate 1, a base layer 2 formed on the substrate 1 by metallization or the like, a dielectric layer 3 formed on the base layer 2, and a conductive microstrip line 4 formed on the dielectric layer 3. With reference to FIG. 1B, a multilayered microstrip including a plurality of the single-layered microstrips shown in FIG. 1A is also used.
Fluoride glass and alumina ceramics have been used for the dielectric layers 3 and the insulating interlayer 5 of such microstrips. In recent years, however, dielectric materials having smaller dielectric constants and smaller signal losses have been required. Japanese Unexamined Patent Application Publication No. 8-228105 discloses a porous ceramic meeting such a requirement.
Japanese Unexamined Patent Application Publication No. 9-213797 discloses a method for making a semiconductor device. In the method, a wet-gel film is formed on a substrate, and a solvent contained in the wet gal film is evaporated by a supercritical or sub-critical drying process to form an aerogel film. Next, the aerogel film is patterned. In more detail, a stock solution containing a metal alkoxide (tetraethoxysilane), a solvent (ethanol), water, and a catalyst is spin-coated onto a rotating substrate. Silicon alkoxide in the stock solution is hydrolyzed to form silicon hydroxide. The silicon hydroxide forms a network of siloxane bonds containing the residual solvents by condensation. That is, a wet-gel film of a silica skeleton containing the solvents is formed on the substrate. When the wet-gel film is dried by a supercritical or sub-critical drying process, a silica aerogel having a low dielectric constant is formed by replacement of the residual solvent with air.
Herein, xe2x80x9csupercritical dryingxe2x80x9d means that a solvent or water is in a supercritical state in which the solvent or water is present as both gas and liquid. The supercritical drying state is achieved under high-temperature and high-pressure conditions which is above the critical point of the solvent. In the supercritical drying process, no gas-liquid interface is formed in the gel. Thus, no stress due to capillary force is applied to the gel skeleton. As a result, the solvent remaining in the gap of the network skeleton can be substantially removed without shrinkage of the wet gel. xe2x80x9cSub-critical dryingxe2x80x9d means drying performed in a sub-critical state which is a high-temperature and high-pressure state near the supercritical state. In the sub-critical drying, the solvent can also be substantially removed without shrinkage of the wet gel. The resulting dried gel (aerogel) film still maintains the network structure of the wet-gel film, and is a porous material having a significantly high porosity and a significantly low relative permittivity. Accordingly, aerogels are expected to be used as new materials for the dielectric layer and the insulating interlayer.
Silicon alkoxide, however, is readily condensed in the presence of a catalyst, that is, is rapidly gelated. Thus, production of aerogel films has the following disadvantages.
When a stock solution containing silicon alkoxide, water, alcohol, and a catalyst is prepared for a wet-gel film in an amount sufficient to coating a plurality of substrates and is spin-coated on these substrates, elapsed times from the preparation of the stock solution to the spin coating differ between these substrates. Since condensation or gelation proceeds in the stock solution immediately after the stock solution is prepared, the viscosity of the stock solution increases over time. When the spin coating is performed at a constant rotation rate, the thickness of the film coated on the substrate increases as the viscosity of the stock solution increases. That is, the thickness of the film on a later substrate is larger than that on an earlier substrate. As the gelation proceeds further, a thin-film cannot be formed on the substrate due to extremely high viscosity. Moreover, the water and the catalytic component in the stock solution will be evaporated during the coating, and thus the gelation will not proceed further. As a result, a high-quality wet-gel film cannot be formed on the substrate.
A possible method for solving the above problem is preparation of stock solutions for the plurality of substrates so that the elapsed times from the preparation to the coating of the stock solutions are the same. Since the viscosities of these solutions are the same during the coating steps, the thicknesses of the films can be equal to each other by fixing the rotation of the substrate.
When the degrees of gelation differ between the substrates at the start of the supercritical or sub-critical drying process, the resulting aerogel films have different porosities even if the wet-gel films have the same thickness.
As a result, the aerogel films do not have uniform, reproducible qualities. For example, aerogel films may be mass-produced by a method shown in FIGS. 2A to 2D. Stock solutions having the same viscosity or the same degree of gelation are applied onto substrates 10 to prepare substrates 12 with wet-gel films 11 having the same thickness, and then these substrates 12 are placed into a holder 13 (hereinafter, a substrate 12 with a wet-gel film is referred to as a xe2x80x9cwet-gel-film-substratexe2x80x9d). After a predetermined number (six in FIG. 2C) of wet-gel-film-substrates 12 is placed into the holder 13, the holder 13 is transferred into a hermetically sealed container 14 for supercritical or sub-critical drying. In this case, for example, the first wet-gel-film-substrate 12a is subjected to the supercritical or sub-critical drying after a longer holding time compared to the last wet-gel-film-substrate 12f. That is, the wet-gel-film-substrates 12a, 12b, . . . , 12f in the holder 13 have different holding times before the supercritical or sub-critical drying. Thus, the degrees of gelation of the wet-gel films on these wet-gel-film-substrates 12a, 12b, . . . , 12f differ from each other at the start of the supercritical or sub-critical drying process, even if the applied stock solutions have the same viscosity or even if the gelation reaction starts after the stock solutions are applied onto the substrates. Such a difference in the degrees of gelation causes differences in porosities of the silica aerogel films obtained by the supercritical or sub-critical drying. Accordingly, the aerogel films obtained by the same supercritical or sub-critical drying process have different properties, e.g., dielectric constant and hydrophobicity. The quality of the aerogel films produced in one lot is, therefore, not uniform.
Accordingly, it is an object of the present invention to provide a method and an apparatus for making an aerogel film having uniform qualities by a supercritical or sub-critical drying process.
It is another object of the present invention to provide a method and an apparatus for simultaneously making a plurality of aerogel films having uniform qualities by a supercritical or sub-critical drying process.
The present inventors have completed the present invention under viewpoints that the gelation reaction is suppressed before supercritical or sub-critical drying to control the degree of gelation of the wet-gel-film-substrate to a predetermined level at the start of the supercritical or sub-critical drying, and that the degree of gelation is measured to determine the start of the supercritical or sub-critical drying.
A method for making an aerogel film in accordance with the present invention includes the steps of performing a gelation reaction of a metal alkoxide on a substrate to prepare a substrate with a wet-gel film, and converting the wet-gel film into an aerogel film by a supercritical or sub-critical drying process of the substrate with the wet-gel film, wherein the degree of gelation of the wet-gel film is controlled to be a predetermined value at the start of the supercritical or sub-critical drying process.
In a preferred embodiment for controlling the degree of gelation of the wet-gel film, the supercritical or sub-critical drying process simultaneously performs supercritical or sub-critical drying of a predetermined number of substrates with wet-gel films which are sequentially produced, and the degrees of gelation of the wet-gel films on the predetermined number of substrates are controlled to be substantially the predetermined value.
In another preferred embodiment for controlling the degree of gelation of the wet-gel film, the gelation reaction is suppressed before the supercritical or sub-critical drying process, and is then released from the suppression or is promoted so as to control the degree of the gelation of the wet-gel film to the predetermined value. In such a case, the gelation reaction is suppressed by maintaining the substrate with the wet-gel film at a low temperature before the supercritical or sub-critical drying process, and is then released from the suppression or is promoted by increasing the temperature of the substrate with the wet-gel film. Alternatively, the gelation reaction is suppressed with a gelation retarder before the supercritical or sub-critical drying process, and is then released from the suppression or is promoted by heating the substrate with the wet-gel film or by irradiating the substrate with the wet-gel film with electromagnetic waves to decompose the gelation retarder. Preferably, the gelation retarder is an organic acid.
In another preferred embodiment for controlling the degree of gelation of the wet-gel film, before the supercritical or sub-critical drying process, the gelation reaction rate of the wet-gel film on the substrate is controlled in response to the time from the start of the gelation reaction of the wet-gel film on the substrate to the start of the supercritical or sub-critical drying process so as to control the degree of the gelation of the wet-gel film to the predetermined value. In this embodiment, the gelation reaction rate is preferably controlled by at least one the type and the amount of at least one reagent of a gelation promoter and a gelation retarder. Alternatively, the supercritical or sub-critical drying process simultaneously performs supercritical or sub-critical drying of a predetermined number of substrates with wet-gel films which are sequentially produced, a holding time from the start of the gelation reaction to the start of the supercritical or sub-critical drying process is calculated for each substrate with the wet-gel film, and at least one of the type and amount of at least one of the gelation promoter and the gelation retarder is changed based on the holding time to control the gelation reaction rate. Preferably, the pH of the gelation promoter is 4 or less or 10 or more at the start of the gelation reaction. More specifically, the gelation promoter is one of hydrochloric acid and ammonia. Preferably, the gelation retarder is an organic acid.
In a preferred embodiment, the degree of gelation of the wet-gel film on the substrate is measured and the supercritical or sub-critical drying process is initiated when the degree of gelation reaches the predetermined value G. Preferably, the degree of gelation of the wet-gel film is determined by measuring the intensity of light absorption or light scattering of the wet-gel film. Alternatively, the supercritical or sub-critical drying process is started after a predetermined elapsed time from the time when the gelation reaction is released from the suppression or is promoted.
In a preferred embodiment, the substrate with the wet-gel film is prepared by applying a stock solution containing the metal alkoxide, water, and a gelation promoter and/or a gelation retarder onto the rotating substrate. Preferably, the degree of gelation of the stock solution is measured and the rotation rate of the substrate is determined in response to the degree of gelation so that the thickness of the wet-gel film is uniform. Preferably, the degree of gelation of the stock solution is measured by intensity of light absorption or light scattering of the stock solution, or the degree of gelation of the stock solution is measured by the viscosity of the stock solution.
In another preferred embodiment, the substrate with the wet-gel film is prepared by supplying the metal alkoxide on the substrate and then supplying water and a gelation promoter and/or a gelation retarder thereon. Preferably, a viscosity modifier is preliminarily added to the metal alkoxide. Preferably, the gelation promoter and/or the gelation retarder are supplied by holding the substrate in an environment containing the gelation promoter and/or the gelation retarder in a gaseous state. Alternatively, the substrate with the wet-gel film is prepared by substantially simultaneously supplying the metal alkoxide and a gelation promoter and/or a gelation retarder onto the substrate. Alternatively, the substrate with the wet-gel film is prepared by mixing and simultaneously supplying the metal alkoxide, water, and a gelation promoter and/or a gelation retarder onto the substrate.
An apparatus for making an aerogel film in accordance with the present invention includes substrate supporting means for supporting a substrate, wet-gel film forming means for forming a wet-gel film on the substrate supported by the substrate supporting means, holding means for holding the substrate with the wet-gel film, supercritical or sub-critical drying means for drying the substrate with the wet-gel film held in the holding means in a supercritical or sub-critical medium, and supercritical or sub-critical drying control means for initiating the supercritical or sub-critical drying by the supercritical or sub-critical drying means when the degree of gelation of the wet-gel film on the substrate held in the holding means reaches a predetermined value.
Preferably, the supercritical or sub-critical drying control means includes a calculation unit for determining the time J when the degree of gelation reaches a predetermined value G, and a first supercritical or sub-critical initiating unit which submits a command for initiating the supercritical or sub-critical drying at the time J. Alternatively, the supercritical or sub-critical drying control means includes a gelation measuring unit for measuring the degree of gelation of the wet-gel film on the substrate held on the holding means, and a second supercritical or sub-critical initiating unit for initiating the supercritical or sub-critical drying when the degree of gelation of the wet-gel film measured by the gelation measuring unit reaches a predetermined value. In such a case, the gelation measuring unit preferably determines the degree of gelation of the wet-gel film by the intensity of light absorption or light scattering.
Preferably, the holding means is provided with temperature controlling means for controlling the gelation reaction rate, or the holding means is provided with electromagnetic wave irradiation means for releasing the gelation reaction from the suppression.
Preferably, the substrate supporting means includes detecting means for detecting the time when the wet-gel film is formed by the wet-gel film forming means, and composition selecting means for determining at least one of the type and amount of a gelation promoter and/or a gelation retarder in response to the detected time.
The wet-gel film forming means may be a type which supplies a stock solution, or may be a type which separately supplies a metal alkoxide and a gelation promoter.
In a preferred embodiment, the wet-gel film forming means includes a stock solution reservoir containing a metal alkoxide, water, and the gelation promoter and/or the gelation retarder, and stock solution supplying means for supplying the stock solution in the stock solution reservoir onto the substrate on the substrate supporting means. In such a case, the wet-gel film forming means may be provided with first composition controlling means for controlling the composition of the stock solution. Preferably, the first composition controlling means determines at least one of the type and the concentration of the gelation promoter and/or the gelation retarder contained in the stock solution reservoir, based on the results selected by the composition selecting means.
Preferably, the substrate supporting means rotates and supports the substrate, and includes gelation measuring means for measuring the degree of gelation of the stock solution in the stock solution reservoir, and rotation controlling means for controlling the rotation rate of the substrate in response to the measured degree of gelation.
In another preferred embodiment, the wet-gel film forming means includes a first reservoir containing a metal alkoxide, a second reservoir containing water initiating the gelation reaction of the metal alkoxide and a gelation promoter and/or gelation retarder for promoting or suppressing the gelation reaction, and supplying means for supplying the metal alkoxide from the first reservoir and the gelation promoter and/or the gelation retarder from the second reservoir onto the substrate. Preferably, the wet-gel film forming means further includes second composition controlling means for controlling at least one of the type and the amount of the gelation promoter and/or gelation retarder in the second reservoir. Preferably, the second composition controlling means controls at least one of the type and the amount of the gelation promoter and/or the gelation retarder in the second reservoir, based on the results selected by the composition selecting means.
According to the method for making the aerogel film, the degree of gelation of the wet-gel film on the substrate is controlled to be a predetermined level before supercritical or sub-critical drying. At the predetermined level, the specific dielectric constant is minimized. Thus, the resulting aerogel film has high qualities, that is, small variations in porosity and specific dielectric constant. When a predetermined number of wet-gel-film-substrates which are sequentially produced are simultaneously subjected to supercritical or sub-critical drying, the degrees of gelation of the wet-gel films are controlled to be substantially the same level at the start of the supercritical or sub-critical drying. Thus, variations in film thickness and film characteristics between substrates can be suppressed. The resulting aerogel films exhibit high qualities, that is, small variations in porosity and specific dielectric constant.
In the method for making the aerogel film of the present invention, the gelation of the metal alkoxide is readily controlled by the types and the amounts of the gelation promoter and/or the gelation retarder.
The apparatus for making the aerogel film according to the present invention can efficiently produce high-quality aerogel films. Since the start of the supercritical or sub-critical drying can be appropriately changed by the predetermined degree of gelation, which depends on the composition of the gelation initiator solution and the gelation reaction rate, this apparatus is applicable to various types of metal alkoxides, gelation promoters, and gelation retarders.